Support for high resistance electrical conductors and method of making the same



EXAMINER 5.5

Patented Feb. 13, 19 51 SUPPORT FOR HIGH RESISTANCE ELECTRI- CALCONDUCTORS AND METHOD OF MAK- ING THE SAME Ate .254 a 4 5 5 /Bat JosephW. Myers, Philadelphia, Pa., assignor to Proctor Electric Company,Philadelphia, Pa., a corporation of Pennsylvania No Drawing. ApplicationOctober 13, 1947, Serial No. 779,636

10 Claims. 1

The present invention relates to a novel support for electric heatingelements and its method of preparation; and more particularly it relatesto a support for high resistance conductors comprising commercialasbestos millboard treated to improve its electrical resistance, heatresistance, flexural strength, hardness, and finish.

In electrical appliances where heat is generated, such as for exampletoasters, heaters, and the like, it is necessary to support theheat-supplying current-carrying resistance elements on a materialcapable of supporting the same and of sufiicient electrical resistanceto prevent the passage of current from the current-carrying heatingelements to the non-current-carrying metal parts of the appliance. It isalso necessary that the support possess sufficiently low thermalconductivity to prevent undue loss of heat from the area intended to getthe full benefit of the heat. In toasters, for example, thecurrent-carrying heating wire is wound back and forth across one face ofa sheetor board-shaped support, spaced projections on both verticaledges of the support serving to hold each 180 bend in the wire. Thesupport in turn is secured to the body of the toaster. In suchappliances, the winding support must be of sufiicient strength andhardness to provide a rigid construction and to withstand bending andcrushing and other abuses encountered in the manufacture and use of suchappliances, and, as stated, it must also possess highelectrical-resistance and low thermal conductivity.

Heretofore, supports for electrical resistance heating wires have beenmade of such materials as mica or ceramic-ware. However, these materials are very expensive and when the area needed to support the heatingwires is large, the cost of providing supports of mica or ceramicmaterials becomes prohibitive. Commercial asbestos mill board hascertain characteristics that would make it appear to be suitable for thepurpose of supporting heating wires since it is somewhat rigid, may becut into desired shapes, and is relatively inexpensive. It was found,however, that commercial asbestos millboard lacks the flexural strengthand hardness desired for this purpose. In addition, it was found thatthe inherent hygroscopicity of the millboard, or, more particularly, ofcertain materials in the millboard, seriously reduces its electricalresistance properties in moist or humid atmospheres. In other words,moisture is absorbed by the millboard causing electrical leakage tooccur from the current-carrying heating elements through the support tothe Kl r a non-current carrying metal parts of the appliance, resultingin the possibility of electric shocks to the user of the appliance.Although this condition exists only until the heat generated by theheating element dries out the absorbed moisture, nevertheless the dangerof electric shock reappears after each period of non-use, and theelectrical leakage is of such a degree that the appliances fail to passthe prescribed tests of acceptance and approval organizations, such asthe Underwriters Laboratories.

An object of the present invention, therefore, is to provide aninexpensive material of high electrical-resistance, high fiexuralstrength, hardness, and low thermal conductivity, even under humidatmospheric conditions, ideally suited as a support for electricalresistance heating elements.

Another object is to provide a treated commercial asbestos millboard ofimproved electricaland heat-resistance properties.

Still another object is to provide a treated commercial asbestosmillboard incapable of absorbing atmospheric moisture, as compared tothe untreated millboard, while possessing greater fiexural strength andhardness.

A further object is to provide a simple method of treating an ordinarilyhygroscopic commercial asbestos millboard to convert it into aheat-stable, non-hygroscopic composition possessing high electricalandheat-insulating properties, high fiexural strength and hardness evenunder humid or moist atmospheric conditions, and an improved finish.

Further objects will be apparent from a consideration of the followingspecification and claims.

In accordance with the present invention, a commercial asbestosmillboard, containing as the main bindi1i'g"matria1s sodium silicate andcalcium compounds selectedf'rom the group consisting of lime and calciumcarbonate, is impregnated with an aqueouys o'lfition of ortho phosphoricacid capable of converting the'cal'cium compounds in the millboard intoheatstaole, non-hygroscopic calcium phosphate and of simultaneouslyconverting the sodiuir gilicate in the millboard into silicic acidTThe'porous millboard readily absorbs the s'olution and thus becomesthoroughly impregnated within a matter of a few seconds. The wetmillboard is then dried, preferably at an elevated temperature, toremove moisture and other volatile substances. The millboard by thistreatment has had the already thoroughly dispersed hydrated lime and/orcalcium carbonate and sodium silicate binders, both on or near thesurfaces of the millboardand internally, converted, in situ, intonon-hygroscopic compounds, namely into calcium phosphate and at leastpartially dehydrated silicic acid.

Referring to the asbestos millboard treated in accordance with thepresent invention, it is generally prepared by water-laying a suspensioncomprising asbestos fibres and binding materials, such as lime andsodium silicate, to form an interfelted web. This web is subsequentlyrolled and dried to form a rigid boardor sheet-like structure knowncommercially as asbestos millboard. The lime in the dried boardgradually becomes converted by the carbon dioxide in the air to calciumcarbonate, which then serves as part of the binding medium. In thetreatment of the present invention; it is immaterial whether some or allof the lime has been converted to calcium carbonate or not, since thetreatment will convert either the hydroxide or carbonate of calcium intonon-hygroscopic, heat-stable calcium phosphate. Other materials such asfillers and other binders such asstargh a d clays may, also be present.The important feature i's', however, the presence of asbestos fibres,and of sodium silicate and calcium compounds, such as calcium carbonateand calcium hydroxide, as the main binding materials; the former toprovide heatand electrical-resistance properties, and the calciumcompounds and sodium silicate for subsequent reaction with thephosphoric acid. I

Such asbestos millboards generally vary in thickness over a'wide range,such as from a few hundredths of an inch to several tenths of an inch inthickness or higher. For the purposes of the present invention, howeverasbestos millboards in the lower range of thicknesses are most suitablesuch as those ranging from about 0.05" to about 0.2. Preferably, themillboard is stamped, pressed, or otherwise cut to its ultimate or finalshape as a support prior to the present treatment, since then the raggedor frayed edges resulting from the cutting will become closely bound tothe surface, by the treatment, and will present a smoother finish.However, the millboard as it is marketed, or in smaller sizes, may alsobe treated in accordance with the present invention after which it maybe cutto its ultimate form. The term millboard will be understood tomean, therefore, the material either in the shape ultimately to be usedas a support or'in any stage of cutting from the millboard as marketedto the ultimate or final shape.

As stated, the above-mentioned millboard is treated with an aqueoussolution of ortho phosphoric acid capable of converting calciumcompounds in the millboard, such as calcium hydroxide and calciumcarbonate, into heat-stable, non-hygroscopic calcium phosphate, and ofconverting sodium silicate into silicic acid. Since the acid will be inexcess, tricalcium phosphate will be formed. As the phosphoric acidreacts with the lime or calcium carbonate, the insoluble calciumphosphate precipitates and, together with the precipitated silicic acid,fills the pores of the millboard. The fiexural strength and hardness ofthe board when dried are thus increased; the surface finish of the boardis improved; and the original relatively porous product is convertedinto one of markedly decreased permeability.

As stated, the treating acid is in the form of an aqueous solution.While solutions of any desired concentrations may be used, generallysolution-s containing between about 50% and of the acid by weight areemployed, preferably between about 35% and about by weight. Theacid-treating solution may be at any temperature below its boilingpoint. Generally, however, from the economics standpoint the solution ismaintained at or near room temperature, for example, between about 60 F.and 90 F. At these ordinary temperatures, the reaction between the acidand the calcium compounds and sodium silicate readily takes place uponimpregnation of the solution into the board, and, therefore, whileelevated temperatures may hasten the reaction somewhat, their use is notnecessary. I

In treating the commercial asbestos millboard with the acid solution,the solution is allowed to thoroughly impregnate the board. This may be7 brought about by spraying; however, dipping or immersing the millboardin the acid solution is the preferred method. The porous absorbentnature of the millboard allows it to readily take up the solution,becoming thoroughly impregnated or saturated with it in a very shorttime. The acid thus contacts the thoroughly and uniformly dispersedsodium silicate and calcium hydroxide and/ or calcium carbonate bindingagents, reacting therewith to form, in situ and at substantially thesame time, silicic acid on the one hand and non-hygroscopic, heat-stablecalcium phosphate on the other. These reaction products, being formedand precipitated in situ and substantially simultaneously from compoundsalready thoroughly and uniformly'dispersed throughout the structure ofthe millboard, are themselves thoroughly and uniformly dispersed thereinto form, when dried, a composition bonded with the heat-stable andnon-hygroscopic compounds.

Once the millboard has become impregnated with the acid solution, it maybe removed from the treating zone, drained, and dried. While the wetmillboard may be dried at ordinary temperatures, at least the majorportion of the drying is preferably ,carried out at elevatedtemperatures, such as above about 212 F. In the preferred embodiment,the wet millboard is allowed to drain at ordinary temperature, in adraft, until liquid ceases to drip from the board. The board is thenplaced in a heating zone such as an oven and heated not only to quicklyexpel moisture and other volatiles, but also to convert the silicic acidinto less hydrated forms of silica by splitting off water. The elevatedtemperature employed in this case is usually above the boiling point ofwater and may be as high as 1200 F., and is preferably between about 400F. and 500 F. The time required to dry the wet millboard depends uponthe thickness and nature of the millboard as well as the temperatureemployed. No difiiculty is encountered in adjusting the propertime-temperature conditions. For example, a saturated millboard ofapproximately 0.075 inch in thickness, after draining, is satisfactorilydried at about 460 F.

in about thirty minutes.

During the drying step, whether it is conducted at room or at anelevated temperature, the moisture in the treated millboard is removedand the resulting silicic acid product in the board is termed herein atleast partially dehydrated silicic acid. Preferably, the drying stepwill be conducted at a temperature above 212 F., not only to shorten thedrying time, but also to convert the silicic acid into silica or aEXAMIEEF slightly hydrated form thereof. both of which are includedherein in the term silica. In the event the drying operation does notconvert the silicic acid into silica, the heat generated by the electricresistance elements during the inspection tests or initial use of theappliance is suflicient to do so.

It has been found that the wet millboard, during drying, tends to bendor warp. In many instances, it is necessary that the support present afiat or plane surface, either when positioned vertically or horizontallyor otherwise. Therefore, it is preferable to secure the wet millboard insuch a position, during drying, as to prevent warping or to keep warpingat a minimum. Suitable supporting frames or other means of maintainingthe millboard in a fiat position, such as clamps, are, therefore,preferably employed during the drying of the wet millboard.

In the event the millboard has not been cut to the form or shapeultimately to be used as the winding support for electrical-resistanceheating elements, previous to the treatment of the present invention, orfor that matter at any time during the treatment, the dried millboardmay be readily stamped, pressgd gr cut into any desired shape. Asstated, however, the millboard is preferably cut into its ultimate orfinal form as a support prior to the treatment of the present inventionsince the cut edges will present a smoother and more closely boundfinish as compared to frayed or ragged edges of a dried millboard outafter the treatment.

The product of the present invention is thus a winding support forelectrical-resistance heating elements in sheetor board-like form cut inany desired pattern. The support comprises water-laid asbestos fibresbound in a thoroughly and uniformly dispersed mixture ofnon-hygroscopic, heat-stable calcium phosphate and at least partiallydehydrated silicic acid. The product is light, relatively inexpensive,and possesses markedly improved hardness and flexural strength and asmoother, harder finish as compared to the untreated commercial asbestosmillboard from which it is produced. Of prime importance, however, isthe fact that the tendency of the treated millboard to absorbatmospheric moisture is negligible, resulting in its correspondinglyhigh electrical-resistance and low thermal conductivity even under moistor humid atmospheric conditions. Furthermore, in the preferredembodiment of the invention, the edges are not ragged and uneven, as arethose of the untreated millboard. It will, therefore, be seen that, bythe relatively simple process of the present invention, a material oflittle value from the standpoint of suitability as a support forelectrical-resistance heating elements has been converted into amaterial highly suited for that purpose.

The following example is given to illustrate the process of the presentinvention and the resulting product, and is not intended to limit thescope of the invention in any way.

Example of 25% by volume of a 75% solution of orthophosphoric acid and75% by volume of water.

for minutes. The pieces were then drained at ductors comprisingcommercial asbestos millatmospheric temperature in a blower draft for 10minutes. After draining, the pieces were secured on racks and the rackswere placed in an oven where the pieces were subjected to circulatingair at a temperature of 400 F. for 30 minutes. The

5 resulting dried products exhibited a smooth, hard,

somewhat glossy surface. The fiexural strength of the product was almostfour times as great as that of the untreated millboard. The product whensubjected to the same humid atmospheric conditions as the untreatedasbestos millboard for a 24-hour period, exhibited an electricalresistance almost forty-two times greater than that of the untreatedmillboard.

I claim:

15 1. A process for producing a support for high resistance electricalconductors which comprises impregnating a commercial asbestos millboardin support form and bonded by sodium silicate and a calcium compoundselected from the group consisting of calcium hydroxide and calciumcarbonate with an aqueous solution of orthophosphoric acid to convertthe sodium silicate into silicic acid and to convert the calciumcompound into heat-stable, non-hygroscopic calcium phosphate, and dryingthe impregnated millboard at a temperature of at least 212 F.

2. The process of claim 1 wherein said im-- pregnated millboard issecured as a fiat plane during drying.

3. A process for producing a support for high resistance electricalconductors which comprises impregnating commercial asbestos millboardbonded by sodium silicate and a calcium compound selected from the groupconsisting of calcium hydroxide and calcium carbonate, with an aqueoussolution of ortho phosphoric acid, and drying the impregnated millboard.

4. A process for producing a support for high resistance electricalconductors which comprises impregnating commercial asbestos millboardbonded by sodium silicate and a calcium compound selected from the groupconsisting of calcium hydroxide and calcium carbonate, with an aqueoussolution of ortho phosphoric acid and drying the impregnated millboardat a temperature of at least 212 F.

5. The process of claim 4 wherein the concentration of ortho phosphoricacid in said solution is between about 10% and about 50%, by weight.

6. The process of claim 4 wherein the concentration of ortho phosphoricacid in said solution is between about 15% and about 35%, by weight.

7. A support for high resistance electrical conductors comprisingcommercial asbestos millboard, initially bonded by sodium silicate and acalcium compound selected from the group consisting of calcium hydroxideand calcium carbonate, treated and modified in accordance with theprocess of claim 1.

8. A support for high resistance electrical conductors comprisingcommercial asbestos millboard, initially bonded by sodium silicate and acalcium compound selected from the group consisting of calcium hydroxideand calcium carbonate, treated and modified in accordance with theprocess of claim 3.

9. A support for high resistance electrical con ductors comprisingcommercial asbestos millboard, initially bonded by sodium silicate and acalcium compound selected from the group consisting of calcium hydroxideand calcium carbonate, treated and modified in accordance with theprocess of claim 4.

10. A support for high resistance electrical con- 7 board. initiallybonded by sodium silicate and a calcium compound selected from the groupconsisting of calcium hydroxide and calcium carbonate, treated andmodified in accordance with the process of claim 5. 5

JOSEPH W. MYERS.

REFERENCES CITED The following references are of record in the file ofthis patent: 10

UNITED STATES PATENTS Number Number FOREIGN PATENTS Country Date GreatBritain 1895

1. A PROCESS FOR PRODUCING A SUPPORT FOR HIGH RESISTANCE ELECTRICALCONDUCTORS WHICH COMPRISES IMPREGNATING A COMMERCIAL ASBESTOS MILLBOARDIN SUPPORT FORM AND BONDED BY SODIUM SILICATE AND A CALCIUM COMPOUNDSELECTED FROM THE GROUP CONSISTING OF CALCIUM HYDROXIDE AND CALCIUMCARBONATE WITH AN AQUEOUS SOLUTION OF ORTHOPHOSPHORIC ACID TO CONVERTTHE SODIUM SILICATE INTO SILICIC ACID AND TO CONVERT THE CALCIUMCOMPOUND INTO HEAT-STABLE, NON-HYGROSCOPIC CALCIUM PHOSPHATE, AND DRYINGTHE IMPREGNATED MILLBOARD AT A TEMPERATURE OF AT LEAST 212* F.